1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for controlling a quantity of fuel supplied into an internal combustion engine to control an air-fuel ratio of an air-fuel mixture toward a target air-fuel ratio, and more particularly to an apparatus for assuring more stable control operation of a feedback system for controlling the air-fuel ratio of the internal combustion engine.
2. Description of Related Art
As an air-fuel ratio control apparatus of this kind, conventionally, there is an apparatus which is disclosed in for instance a Japanese laid-open patent publication No. 1-110853. Namely, in this apparatus a so-called modern control theory is also utilized in which taking basically into account the dynamic model of a system for controlling an air-fuel ratio of an internal combustion engine, a quantity of fuel to be supplied on each occasion is feedback-controlled under an optimal gain wherein an internal state of the model is estimated. In such a modern control theory, construction of a state observing instrument called as an observer is generally needed, and its control quantity and control scale become enormous. In view of this reason, especially in this apparatus air-fuel ratio control means is constructed by the dynamic model of the internal combustion engine for determining an air-fuel ratio, approximating a dead-time by an auto-regressive model whose model order is 1 and further taking into account the disturbance. The the apparatus is provided with: a state variable quantity output part for outputting an air-fuel ratio of the internal combustion engine and a control quantity of fuel supply quantity control means as a state variable quantity representing the internal state of the dynamic model of the internal combustion engine; an accumulation part for accumulating a deviation between a target air-fuel ratio and an actual detected air-fuel ratio; and a control quantity calculation part for calculating a control quantity of a fuel supply quantity control means from an optimal feedback gain predetermined based on the dynamic model, the state variable quantity and the accumulated value by the accumulation part. Accordingly, this makes such an observer unnecessary.
In such construction the aforementioned model must be always constructed with a dynamic model which exactly satisfies a relation between a fuel supply quantity to the internal combustion engine and an air-fuel ratio of the air-fuel mixture.
In an air-fuel ratio sensor constructing the aforementioned air-fuel ratio detection means, however, there is in general a tendency that an output at the rich side is difficult to be produced as compared with an output at the lean side. Accordingly, there is a possibility that the above-mentioned model relationship is changed in these ranges, that is, concretely in ranges wherein the sensor outputs may not take a linear value. If the dynamic model relationship is thus changed, overcorrection or correction shortage is caused by a deviation between the actual air-fuel ratio and the aforementioned detected air-fuel ratio, and besides there is a possibility of hunting being caused.
In addition, such an air-fuel ratio sensor has a state that an output of the air-fuel ratio sensor changes slowly with respect to oxygen concentration of the exhaust gas and a state that the output changes rapidly with predetermined oxygen concentration bordered. In view of this fact, in an air-fuel ratio control apparatus disclosed in for instance a Japanese laid-open patent publication No. 62-248848, a predetermined value is compared with a difference between outputs of the air-fuel ratio sensor corresponding respectively to the actual air-fuel ratio and a target air-fuel ratio to switch over an output state of the air-fuel ratio sensor. Then, feedback control of the air-fuel ratio is performed on a basis of a relative output state (a state in which an electric voltage is applied across electrodes) of the same sensor until the detected result becomes substantially stable. After the detected result has stabilized, feedback control of the air-fuel ratio is performed on a basis of an output in the rapidly changing output condition (a state in which the electric voltage is not applied across the electrodes) of the same sensor. In this case, however, there is still no change in the fact that an output at the rich side of the air-fuel ratio sensor is difficult to be produced as compared with an output at the lean side. Especially at the rich side, therefore, it is also doubtful whether or not the output of the aforementioned relative output state itself correlates exactly with oxygen concentration of the exhaust gas. If such correlation has been changed, the control performed based on the detected output of the air-fuel ratio becomes also low in its reliability naturally. This is likely to cause hunting in an extreme case.
In addition to the aforementioned problems, an oxygen concentration sensor which ordinarily generates a rich output or a lean output is activated at about 300.degree. C., while in an air-fuel ratio sensor constructing the aforementioned air-fuel ratio detection means a rich/lean output corresponding to a limiting electric current or output of the air-fuel ratio sensor is initiated at 400.degree. C. Usually, the air-fuel ratio sensor must be used at temperature wherein the limiting current is stabilized. The element temperature of about 630.degree. C. is thus needed for measuring the air-fuel ratio over a range till the temperature reaches 400.degree. C. Accordingly, it is impossible to start feedback of the air-fuel ratio till the element temperature reaches about 630.degree. C. As a result, the starting timing of the feedback is delayed considerably as compared with normal feedback of the air-fuel ratio given by the oxygen concentration sensor, and the drawback occurs that HC in the exhaust gas becomes bad.
As a countermeasure, proposed is a method that when the limiting electric current starts to be produced, the feedback is started at once. However, the output characteristic of the air-fuel ratio sensor does not stabilize in a semi-warming up state of the above-mentioned air-fuel ratio sensor. Accordingly, the dynamic model relationship between the fuel injection quantity and the air-fuel ratio is changed. As a result, there occurs the drawback that control of the air-fuel ratio expected by use of the modern control theory may not be realized properly. Especially, in the air-fuel ratio control apparatus of the aforementioned construction, the air-fuel ratio control becomes unstable increasingly due to high responsibility. Thus, there is a possibility of hunting phenomena.
In view of such an output characteristic of the air-fuel ratio sensor, there is an apparatus which determines whether or not an air-fuel ratio sensor is maintained in a stable state. Then, feedback control of an air-fuel ratio based on an output of the air-fuel ratio sensor is performed if the air-fuel ratio sensor is maintained in the stable state, whereas quasi-data stored in advance in a memory is substituted for the control if the air-fuel ratio sensor is maintained in an unstable state (refer to a Japanese laid-open patent publication No. 1-219327). Furthermore, there is an apparatus which determines whether or not the air-fuel ratio sensor is maintained in a stable state likewise. Then, feedback control is performed so as to make the actual air-fuel ratio a desired air-fuel ratio except the stoichiometric or theoretical air-fuel ratio if the air-fuel ratio sensor is maintained in the stable state, whereas the feedback control is performed so as to make the actual air-fuel ratio the stoichiometric air-fuel ratio if the air-fuel ratio sensor is maintained in an unstable state (refer to a Japanese laid-open patent publication No. 60-27749. However, there is room for further improvement in these apparatuses from a point of view of flexibility along an internal condition of the actual control system and responsiveness (a converging speed) of a feedback system.